It’s British and means your jaw hit the floor and stayed there because you were too astonished to close your mouth.

Reader:            I was gobsmacked when I read your blog.

Yearstricken:   I’m so glad. I really enjoy gobsmacking others.

Reader:            Yes, there’s really nothing quite as bracing as a good gobsmack.

Yearstricken:   Well, tally ho and all that, I must get back to my blog. There are still so many gobs to smack.